Ammonia is the molecule NH₃ — a pungent gas, often sold dissolved in water as a cleaner. Ammonium is the ion NH₄⁺, what ammonia becomes when it gains a hydrogen ion. Ammonia is a neutral molecule and can be a gas; ammonium is charged and exists only in solution or in salts.
The core difference
- ammonia — NH₃, a neutral molecule. A colourless gas at room temperature with a sharp, unmistakable smell.
- ammonium — NH₄⁺, a positively charged ion formed when ammonia picks up a proton.
One hydrogen and one unit of charge separate them, and the pair converts back and forth depending on how acidic the surroundings are.
Which one you have depends on pH
In acidic or neutral water, ammonia mostly takes up a proton and exists as ammonium. In alkaline conditions the balance shifts back to ammonia, which can then leave the solution as gas — which is why an alkaline cleaner smells strongly and a neutral one may not.
This matters practically in aquariums and in agriculture: ammonia is far more toxic to fish than ammonium, so the same total concentration is more dangerous in alkaline water.
Where each appears
- Ammonia — household cleaners (as ammonium hydroxide solution), refrigerants, and as feedstock for fertiliser manufacture.
- Ammonium — in salts: ammonium nitrate and ammonium sulfate as fertilisers, ammonium chloride in batteries and in salty liquorice.
The safety point
Mixing ammonia-based cleaners with bleach produces chloramine gases, which are toxic. This is the second half of the standard bleach warning, alongside not mixing bleach with acids.
The name
From sal ammoniac, ammonium chloride, said to have been collected near the temple of Amun in ancient Libya — the same root that gives ammonite, the fossil whose coiled shape recalled the ram's horns of the god.
Frequently asked questions
What is the difference between ammonia and ammonium?
Ammonia is NH₃, a neutral molecule that can be a gas. Ammonium is NH₄⁺, the charged ion it becomes when it gains a hydrogen ion, and exists in solution or in salts.
Why does pH matter?
In acidic or neutral conditions ammonia mostly becomes ammonium; in alkaline conditions it reverts to ammonia and can escape as gas. Ammonia is far more toxic to fish than ammonium.
Why should ammonia not be mixed with bleach?
The combination produces toxic chloramine gases. It is the second half of the standard bleach warning, alongside not mixing bleach with acids.